The present invention generally relates to a liquid crystal optical switching device and, in particular, relates to such a switching device including a liquid crystal beam splitter.
The use of optical fibers as a signal transmission medium has been demonstrated to exhibit numerous advantages compared to current transmission media, such as coaxial cable and microwave links. In a practical implementation a typical telecommunication system requires hundreds, if not thousands, of signal switches in order to serve the vast number of subscribers thereto. Conventionally, the optical switches most frequently used have been mechanical in nature.
Mechanical switches inherently include moving parts and generally require high driving power. As such, mechanical switches are inherently subject to wear, abrasion, fatigue and other mechanical stresses. As a consequence, mechanical switches are prone to failure after repeated use.
Recently, optical switches utilizing a liquid crystal material as the optical signal direction control mechanism have been proposed. However, at the present time, these proposed liquid crystal switches are both expensive and not amenable to mas production techniques.
Typical of these proposed liquid crystal switches are those discussed and described in U.S. Pat. No. 4,201,442 issued to McMahon et al. on May 6, 1980, U.S. Pat. No. 4,278,327 issued to McMahon et al. on July 14, 1981 and U.S. Pat. No. 4,385,799 issued to Soref on May 31, 1983. All of these devices require the use of a pair of trapezoidal prisms, each prism having four optically flat surfaces. Such prisms are difficult to manufacture and certainly represent a considerable expense.
In addition, such liquid crystal switches, during the assembly thereof, require that the optically flat bases of the prisms be parallel to each other and laterally aligned to ensure that the path of the light beams passing therethrough remain as stable as possible.
Further, the devices described in the above referenced patents are difficult to manufacture since each port requires individual critical alignment. Still further, since there are index of refraction mismatches at the prism/electrode interface and the electrode/liquid crystal material interface, considerable losses are introduced. These mismatches also result in excessive crosstalk between, for example, the parallel and perendicular polarizations of an inputted plane polarized light signal. Since such a light beam traverses, or is reflected by, these interfaces at least twice before exiting the device, any crosstalk due to the mismatched indices of refraction mismatches is compounded. In addition, the diffusion of the light beam and the possibility of internal scattering due to internal impurities increases with the path length within the prism.
Nevertheless, all such liquid crystal switches exhibit the advantage of no moving parts and consequently avoid numerous difficulties associated with mechanical switches.
Improved liquid crystal optical switching device that are readily mass produced are discussed and described in U.S. patent application Ser. No. 795,156 entitled LIQUID CRYSTAL OPTICAL SWITCHING DEVICE and U.S. patent application Ser. No. 795,151 entitled LIQUID CRYSTAL OPTICAL SWITCHlNG DEVICE HAVING MINIMIZED INTERNAL LIGHT PATH, both filed on even date herewith and assigned to the assignee hereof. These applications are deemed incorporated herein by reference. One particular drawback of the devices described in these references, is that, by themselves, they are capable of switching only a single polarization of an incident light beam. As discussed therein, this drawback can be overcome by integrating two such devices into a single apparatus and switching one of the devices to effect the optical path redirection. Nonetheless, as with any liquid crystal optical switching device, the increased light path length tends to increase crosstalk between the various ports due to such phenomena as beam diffusion, loss of polarization purity as well as other known optical effects.
Consequently, in order to fully realize the potential advantages of an optical communications system, an optical switch that provides reduced crosstalk, i.e., increased isolation between ports, is relatively inexpensive in the materials used to manufacture that switch and is amenable to mass production techniques is clearly needed.